EnrichProcessorFactory should not throw NPE if missing metadata (elastic#55977) (elastic#56843)

In some cases the Enrich processor factory may be called before it is
ready to create processors. While these calls are usually made in error,
the response from the Enrich processor is an NPE which is almost always
an unhelpful error when debugging an issue.

Author: jbaiera

x-pack/plugin/enrich/src/main/java/org/elasticsearch/xpack/enrich/EnrichProcessorFactory.java

@@ -38,6 +38,9 @@ final class EnrichProcessorFactory implements Processor.Factory, Consumer<Cluste
     public Processor create(Map<String, Processor.Factory> processorFactories, String tag, Map<String, Object> config) throws Exception {
         String policyName = ConfigurationUtils.readStringProperty(TYPE, tag, config, "policy_name");
         String policyAlias = EnrichPolicy.getBaseName(policyName);
+        if (metadata == null) {
+            throw new IllegalStateException("enrich processor factory has not yet been initialized with cluster state");
+        }
         IndexAbstraction indexAbstraction = metadata.getIndicesLookup().get(policyAlias);
         if (indexAbstraction == null) {
             throw new IllegalArgumentException("no enrich index exists for policy with name [" + policyName + "]");